Pulsed laser source with high repetition rate

ABSTRACT

Methods and systems for generating pulses of laser radiation at higher repetition rates than those of available excimer lasers are disclosed that use multiple electronic triggers for multiple laser units and arrange the timings of the different triggers with successive delays, each delay being a fraction of the interval between two successive pulses of a single laser unit. Methods and systems for exposing nanoscale patterns using such high-repetition-rate lasers are disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to pulsed laser source systems, and particularlyrelates to methods and apparatus for ultraviolet, pulsed, excimer lasersource systems with high repetition rates. Such laser systems are usefulas light sources in nanolithography systems for production of electronicdevices.

2. Description of Related Art

The manufacture of modern electronic devices, commonly referred to asintegrated circuits (ICs) or chips, requires a number of fabricationtechnologies. One of the most critical of such fabrication technologiesis lithography, the process of patterning the billions of structuresthat form the individual components of the devices on the semiconductorwafers. Advances in the manufacture of electronic devices have requiredthe patterning of ever smaller structures on the wafers, which, for theprocess of lithography, is referred to as requiring higher (i.e., finer)patterning resolution.

A key element in a lithography system that enables it to achieve a finepatterning resolution is its light source, which in modern lithographysystems is an ultraviolet excimer laser due to its short wavelength.Typically, modern lithography systems use an Argon Fluoride (ArF)excimer laser source that emits radiation of 193 nanometer (nm)wavelength. Due to the fundamental physical operating mechanism of sucha laser, it operates only as a pulsed source, with a typical pulserepetition rate of a few hundred to a few thousand pulses per second.

A modern lithography system with an excimer laser source also comprisesa high-resolution, large-field projection lens that creates an image ofa master pattern present on a mask onto the semiconductor wafer. Theoverall performance of the lithography system is determined by theprojection lens, the light source, the mask, and several other factors.Current state-of-the-art lithography systems are capable of producingdevice structures in high volumes with a minimum feature size in thevicinity of 22-45 nm. With such small feature sizes, electronic chipswith several billion transistors can be produced.

The demands on electronic systems to operate at ever greater speeds andhave ever greater storage capacities are requiring more advanced chipswith minimum feature sizes smaller than 22 nm. Modern lithographysystems are incapable of patterning electronic structures with suchsmall features with sufficiently high production throughputs forrequired cost efficiencies. There is thus a need to develop advancedlithography systems that can provide a patterning resolutionsignificantly finer than 22 nm and patterning throughput of, forexample, 100 or more wafers per hour. Such lithography systems arecurrently not available. To meet these objectives, many new lithographyapproaches are being investigated in the semiconductor industry and atresearch institutions, including extreme ultraviolet lithography,maskless lithography, immersion lithography, and other.

Of these new approaches, maskless lithography holds particularly strongpromise due to its many advantages, including high resolution andelimination of the mask as a requirement in the lithography process.(That the latter is significant can be recognized by noting that thecost of the mask set for patterning the layers of a modern chip exceedsfive million dollars.) Examples of methods and apparatus for masklesslithography are disclosed in U.S. Pat. No. 6,312,134, Seamless, MasklessLithography System Using Spatial Light Modulator, 2001; U.S. Pat. No.6,707,534, Maskless Conformable Lithography, 2004; U.S. Pat. No.6,870,554, Maskless Lithography with Multiplexed Spatial LightModulators, 2005; and U.S. Pat. No. 7,164,465, Maskless Lithography withSub-Pixel Resolution, 2004.

In a maskless lithography system, the conventional hard mask as used ina typical optical projection lithography system is replaced by a spatiallight modulator (SLM) array. Each element (i.e., individual element) inthe SLM array can be programmed to be “On” or “Off”, i.e., reflective ornonreflective for a reflective-type SLM (or transmissive ornontransmissive for a transmissive-type SLM), so that the collection ofall the beams emerging from an SLM array can be programmed to representany desired pattern of light pixels that can then expose aphotosensitive medium to create the corresponding pattern therein.

State-of-the-art SLMs have modulator elements of size in the vicinity of10 micrometer×10 micrometer. In a maskless lithography system, by usinga projection lens with a reduction ratio of approximately 200:1, animage pixel size of (10 micrometer)/(200)=50 nm can be produced. Thus,in order to improve the resolution of a maskless lithography system, themodulator element size must be reduced or the projection lens reductionratio must be increased, both of which avenues are difficult.

It will therefore be beneficial to devise a technique that provideshigher resolution for a maskless lithography system than the minimumfeature size (“pixel size”) printed on the basis of the SLM element sizeand the projection lens reduction ratio.

Methods and apparatus for maskless lithography for providing aresolution finer than a pixel size, i.e., sub-pixel resolution, havebeen developed and are disclosed in U.S. Pat. No. 6,717,650, MasklessLithography with Sub-Pixel Resolution, 2004, and U.S. Pat. No.7,170,669, Spatial Modulator with Minimized Heat Absorption and EnhancedResolution Features, 2007. These methods and apparatus definesub-pixel-size features by partial overlap between pixel-size features,exploit nonlinear photoresponse characteristics of the imaging media,and effectively use massively parallel bit addressing for full-patterndefinition and high throughput.

In addition to the above considerations, the performance achievable bymaskless lithography systems is dependent not only upon the ability ofthe SLM to rapidly transfer the pattern information from the data fileto the imaging medium, but also upon the ability of the light source toilluminate the SLM with a new pulse every time the SLM frame (i.e., thearray of all the modulator elements) is refreshed (i.e., provided a newset of pattern data). Modern SLMs can have frame refresh rates as highas 25 kHz, i.e., all the modulator elements can be provided with new“On” or “Off” information 25,000 times per second. In order to utilizesuch a high frame refresh rate capability, the light source must also beable to provide the same number of pulses per second. Modern excimerlaser light sources are available with pulse repetition rates that arelimited to approximately 6 kHz. Available light sources are thereforeinadequate for implementation in maskless lithography systems with thehighest refresh rate SLM arrays.

Therefore, there is a need to develop an ultraviolet excimer laser lightsource capable of providing pulses at repetition rates in the vicinityof 25 kHz and preferably even higher.

It is an object of this invention to provide a method for producingpulsed ultraviolet laser radiation at high repetition rates.

It is another object of this invention to provide an apparatus forproducing pulsed ultraviolet excimer laser radiation with repetitionrates of tens of thousands of pulses per second.

It is yet another object of this invention to provide a high-resolutionmaskless lithography method and apparatus utilizing ahigh-repetition-rate laser light source for illuminating a spatial lightmodulator array.

With the above examples of objects, other objects of this invention willbe evident to those skilled in the art of semiconductor manufacturing,lithography, and related fields.

An advantage of the invention is that it enables effective utilizationof high-refresh-rate spatial light modulators in maskless lithographysystems for achieving high throughputs and high resolutions.

Another advantage of the invention is that it provides the ability toproduce high-repetition-rate laser pulses using lower-repetition-ratelaser sources.

Yet another advantage of this invention is that it enables theoptimization of the combined operation of the illumination source andthe spatial light modulator array in a maskless lithography system toachieve optimum throughput and resolution.

With the above examples of the advantages, other advantages of thisinvention will be evident to those skilled in the art of semiconductormanufacturing, lithography, and related fields.

LIST OF DRAWINGS

FIG. 1 illustrates the basic concept of the invention, showing multipleexcimer laser units, each unit having a master-oscillator sub-unit and apower-amplifier sub-unit, each laser unit triggered by well definedpulse triggers, such that specified delays introduced between thetrigger pulses produce a final laser pulse train that has asubstantially higher effective repetition rate than possible with asingle laser unit.

FIG. 2 shows the basic concept of maskless lithography and identifiesthe key components of such a lithography system, including a lightsource, a spatial light modulator array, a projection lens, and ascanning stage.

FIG. 3 illustrates the operation of a spatial light modulator array,showing how individual modulators can be turned on or off as requiredfor exposure of the desired pattern directly on the semiconductor wafer.

FIG. 4 shows a representative excimer laser system configuration called“master-oscillator-power-amplifier” (MOPA) that comprises two sub-units,called a master oscillator (MO) and power amplifier (PA).

FIG. 5 illustrates the arrival times of the pulses from the differentMOPA units with the relative delays between them of T2, T3, and T4, eachof which is a fraction of the interval T between successive pulses froma MOPA unit.

FIG. 6 illustrates the arrival times of the pulses from four differentMOPA units, each of which has an interval 160 ps between successivepulses, with the relative delays between them of 40, 80, and 120 μs,which are one-fourth, half, and three-fourths of the interval betweensuccessive pulses from a single MOPA unit.

FIG. 7 illustrates an embodiment of the invention in which the arrivaltimes of the pulses from the different MOPA units are made variable bymaking the relative delays between them of T_(b), T_(c), and T_(d)variable.

FIG. 8 shows an embodiment of the invention in which the arrival timesof the pulses from the different MOPA units are dynamically optimized byutilizing information about the pattern to be exposed and the spatiallight modulator elements to be illuminated.

FIG. 9 illustrates an embodiment of the invention in which the arrivaltimes of the pulses from the different MOPA units are made variable bymaking the relative delays between them of T_(b), T_(c), and T_(d)variable directly by signals from a computer.

FIG. 10 illustrates a maskless lithography system comprising ahigh-repetition-rate laser source, a spatial light modulator array, aprojection lens, other optical components, a scanning stage, and controlcomputer that utilizes detailed pattern information to optimize andinteractively control all subsystems.

SUMMARY OF THE INVENTION

In this invention, it is disclosed how an excimer laser source systemcan be constructed that has a higher pulse repetition rate than that ofavailable excimer lasers. The fundamental concept of the invention,illustrated in FIG. 1, is to use multiple electronic triggers formultiple excimer laser units and arrange the timings of the differenttriggers with successive delays, each delay being a fraction of theinterval between two successive pulses of a single laser unit.

The basic concept of maskless lithography is well known (prior art).Conceptually, a representative optical maskless lithography system (FIG.2) uses an ultraviolet laser beam, e.g., from an excimer laser, toilluminate a spatial light modulator, e.g., a digital micromirror device(DMD), which is a 2-D array of micromodulators. Acting as reflectors,these hundreds of thousands (or millions) of modulators direct a desiredset of beamlets into a high-reduction-ratio projection lens, whichimages the modulators on a photoresist-coated wafer, which can bestepped or scanned. In brief, the key features and attributes of such amaskless lithography are the following: (i) The conventional hard maskused in optical projection exposure tools is replaced by a spatial lightmodulator (SLM). (ii) The SLM is a 2-D array of reflective ortransmissive light modulators which can independently control 10⁶ oreven >10⁷ light beams. (iii) A desired set of beamlets reflected fromthe SLM (or transmitted beamlets if it is a transmissive SLM) aredirected into a projection lens (FIG. 3). These “On” beams correspond tolocations where exposure in the resist is desired. The other (“Off”)beams are not captured by the projection lens.

Currently, the most advanced types of SLMs are the digital micromirrordevice

(DMD) chips, among which a leading device has approximately two millionmodulator elements and a “frame refresh rate” of approximately 23 kHz,which is the rate at which the DMD chip frame, i.e., all the mirrorelements, can be sent new bit information. In order to achieve as high awafer exposure rate as possible, the lithography system must utilize themaximum frame refresh, which requires that the light source be able toilluminate the SLM with at least one new pulse for each frame. Thus, theillumination source, i.e., the ultraviolet laser, must be able to emitpulses at the rate of at least 23 kHz. Often, it is desirable that eachSLM frame be illuminated by two or more pulses to achieve more preciseexposure control. In such a case, the illumination source must be ableto provide pulses at a rate that is a multiple of the SLM frame refreshrate.

The most desirable light source for high-resolution lithography is anultraviolet excimer laser due to its short wavelength, high power, andother favorable characteristics. For example, many modern lithographysystems for commercial production of semiconductor devices use an ArgonFluoride (ArF) excimer laser emitting pulsed radiation at a wavelengthof 193 nm. It will be therefore desirable also to develop a masklesslithography system that can use an ArF excimer laser as the lightsource. In the ArF excimer laser active medium which contains a mixtureof fluorine, argon and a buffer gas, the laser photon is emitted when anelectron falls from an excited metastable state of the rare gas halide(ArF) to an unstable state of that halide. Due to the nature of thisfundamental operating mechanism of an excimer laser, it can emit laserradiation only in a pulsed manner. The highest rate at which laserpulses can be emitted by modern excimer lasers is limited to a fewthousand pulses per second, with a maximum rate of approximately 3-6kHz. Since modern SLMs can have frame refresh rates of several timesthat value, it is desirable to develop methods that can significantlyincrease the pulse repetition rates of excimer lasers, as described inthis invention.

A typical laser comprises an optical resonator with two mirrors betweenwhich the active medium is contained and between which the light raysbounce back and forth, enabling the laser beam to build up by stimulatedemission of radiation. A representative excimer laser system, due tohigh gain in its active lasing medium, can be constructed with anoptical resonator that has only one mirror, the other mirror beingreplaced by transmitting window. While such a configuration provideshigh laser power output, it may also broaden the frequency bandwidth ofthe laser radiation, which is sometimes not desirable. In order toproduce a laser beam with better spectral characteristics, an alternateconfiguration for excimer laser systems comprises two sub-units, eachbeing essentially a separate excimer laser, called a master oscillator(MO) and a power amplifier (PA), as shown in FIG. 4. In such aconfiguration, called master-oscillator-power-amplifier (MOPA), the MOunit has an optical resonator with two mirrors and produces a laser beamwith a very narrow spectral bandwidth, typically less than 0.5picometer, and low pulse energy. The beam from the MO is then used to“seed” a PA unit, which has no resonator mirrors (both mirrors beingreplaced by transmitting windows) and whose function is to amplify thebeam from the MO while maintaining its spectral characteristics. Theresulting laser beam emerging from the MOPA therefore has both thenarrow bandwidth produced by the MO and the high pulse energy producedby the PA. However, as explained in the preceding paragraph, it does nothave the desired high pulse repetition rate needed to utilize the highframe refresh rates of modern SLMs.

In this invention, an excimer laser source system is disclosed that hasa higher pulse repetition rate than that of available excimer lasers. Inits key concept, the invention uses multiple electronic triggers formultiple excimer laser units and arranges the arrival times of thedifferent triggers with successive delays such that each delay is afraction of the interval between two successive pulses of a single laserunit. For example, consider an excimer laser with a pulse repetitionrate of 2 kHz. The interval between two successive pulses from such alaser is 500 microseconds. If now one uses five lasers and triggers themby pulses that are successively delayed by 100 microseconds, then thefive lasers taken together will produce 10,000 laser pulses per secondthat are emitted at intervals of 100 microseconds, i.e., effectively,providing an excimer laser source with a pulse repetition rate of 10kHz.

In the excimer laser active medium, a high-voltage electric discharge isproduced by a high-voltage pulse generator, such as a thyratron, that istriggered by low-voltage pulses from a conventional electronic pulsegenerator. In this invention, such a pulse generator serves as a masterpulse generator. Using the above example, each pulse from the generatoris split into five pulses. Of these, pulse no. 1 triggers a MOPA unitdirectly; pulse no. 2 passes through an electronic delay unit thatdelays its arrival by 100 microseconds and triggers the discharge of aMOPA system no. 2; pulse no. 3 passes through an electronic delay unitthat delays its arrival by 200 microseconds and triggers the dischargeof a MOPA system no. 3; and so on, until pulse no. 5 which passesthrough an electronic delay unit that delays its arrival by 400microseconds and triggers the discharge of a MOPA system no. 5.

The pulses emerging from the five MOPA units, considered together, theneffectively provide a train of excimer laser pulses arriving every 100microseconds, i.e., at the rate of 10 kHz. As will be immediately clear,this configuration can be varied in many different ways to provide anexcimer laser source system with many different desirable pulserepetition schemes.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention is shown in FIG. 1. The systemillustrated in FIG. 1 is an Argon Fluoride excimer laser system thatproduces laser pulses of 193 nm wavelength at a high repetition rate,for example, 25 kHz. The system can similarly be a KrF or other excimerlaser. It comprises multiple sets (for example, four) ofmaster-oscillator-power-amplifier (MOPA) units (100, 200, 300, 400).Each MOPA unit (prior art) comprises a master-oscillator (MO) sub-unitand a power-amplifier (PA) sub-unit. For example, MOPA 1, denoted as100, comprises an MO 101 and a PA 102. Similarly, MOPA 2 (200) comprisesan MO 201 and a PA 202, etc. Each of the MOs is an excimer laser havingan optical resonator. For example, MO 101 is an excimer laser comprisingactive lasing medium 103, electrodes 104 and 105, and resonator mirrors106 and 107.

Each MO produces an excimer laser “seed” beam with narrow spectralbandwidth and low pulse energy. For example, MO 101 produces an excimerlaser seed beam 120 with a spectral bandwidth of a fraction of apicometer (pm) and a pulse energy of a few microjoules (μJ). Theresonator mirror 106 is a high-reflectivity mirror with a flat orconcave surface. The mirror 107 has a lower reflectivity than that ofmirror 106, and may have a flat or concave surface. The active medium103 is a mixture of various gases, such as argon, fluorine, and a buffergas, such as helium. The electrodes 104 and 105 produce a high-voltageelectric discharge in the active lasing medium 103. The electrode 104receives a high-voltage pulse 108 from a high-voltage pulse generator109. Electrode 105 is typically grounded. Pulse 108 may be, for example,a 30 kilovolt (kV) pulse and may have a pulsewidth on the order of amicrosecond (μs). The high-voltage pulse generator 109 comprises, forexample, a thyratron, and is triggered by a trigger 110 from a typicallow-voltage electronic pulse generator 10. The pulse rate provided bythe pulse generator 10 may be, for example, 6.25 kHz, which then willalso be the repetition rate at which laser pulses are emitted by MO 101as seed beam 120.

The power amplifier 102 comprises active lasing medium 111, electrodes112 and 113, and transmissive windows 114 and 115 which have flatsurfaces. The active medium 111 is a mixture of various gases, such asargon, fluorine, and a buffer gas, such as helium. The electrodes 112and 113 produce a high-voltage electric discharge in the active lasingmedium 111. The electrode 112 receives a high-voltage pulse 116 from ahigh-voltage pulse generator 117. Electrode 113 is typically grounded.Pulse 116 may be, for example, a 30 kV pulse and may have a pulsewidthon the order of a μs. The high-voltage pulse generator 117 comprises,for example, a thyratron, and is triggered by a trigger 110 from atypical low-voltage electronic pulse generator 10 in synchronism withthe triggering of the pulse generator 109. The excimer laser seed beam120 emitted by MO 101 is amplified by PA 102, resulting in the finallaser output beam from MOPA 100, denoted as beam 121. The spectralbandwidth of beam 121 is substantially the same as the spectralbandwidth of beam 120 and may be, for example, a fraction of apicometer. The energy of each pulse in beam 121 is significantly greaterthan the energy of each pulse in beam 120 and may be, for example, a fewmillijoules (mJ).

In an alternate configuration, the master oscillator (MO 101) may be anultraviolet laser source other than an excimer laser. For example, apulsed rare gas ion laser, or a pulsed tunable dye laser, or a pulsedtunable solid-state laser may be frequency-multiplied in a nonlinearoptical medium to generate narrow-bandwidth ultraviolet laser radiationat the same wavelength as an excimer laser and may thus serve as theseed laser for the power amplifier (PA 102) which is an excimer laser.

MOPA 200 is nearly identical (but not entirely) to MOPA 100 andcomprises MO 201 and PA 202 which are triggered by, respectively, pulses208 and 216 from high-voltage pulse generators 209 and 217, both ofwhich in turn are triggered synchronously by pulse 210. The differencebetween MOPA 100 and MOPA 200 is that the low-voltage trigger pulse 210is not synchronous with trigger pulse 110. Rather, trigger pulse 210 isdelayed after 110 by a certain delay interval T2. The pulse 210 isproduced by splitting the output signal 150 from the low-voltage pulsegenerator 10 into as many signals as there are MOPA units. For theexample of FIG. 1, the output signal 150 is split into four signals,110, 250, 350, and 450. As already described in the precedingparagraphs, the signal 110 triggers the high-voltage pulse generators109 and 117 of MOPA 100. The signal 250 goes through an electronic delayunit 260 which produces the pulse 210 that is delayed after pulse 110 bya time interval T2. Since pulse T2 triggers the high-voltage pulsegenerators 209 and 217, pulses in the output beam 221 of MOPA 200 aredelayed from pulses in the output beam 121 of MOPA 100 by the same timeinterval T2. We remark that electronic delay units such as 260, 360, and460 are readily available as electronic instruments that are commonlyused.

In a like manner, as illustrated in FIG. 1, the signal 350 goes throughan electronic delay unit 360 which produces the pulse 310 that isdelayed after pulse 110 by a time interval T3. Since pulse T3 triggersthe high-voltage pulse generators 309 and 317, pulses in the output beam321 of MOPA 300 are delayed from pulses in the output beam 121 of MOPA100 by the same time interval T3. Similarly, the signal 450 goes throughan electronic delay unit 460 which produces the pulse 410 that isdelayed after pulse 110 by a time interval T4. Since pulse T4 triggersthe high-voltage pulse generators 409 and 417, pulses in the output beam421 of MOPA 400 are delayed from pulses in the output beam 121 of MOPA100 by the same time interval T4. The output beams 121, 221, 321, and421 from MOPA units 100, 200, 300, and 400, respectively, and theirrelative arrival times and delays are further illustrated in FIG. 5.Note that each of the delays T2, T3, and T4 is a fraction of theinterval T between successive pulses of a MOPA, and T4>T3>T2.

As an example, illustrated in FIG. 6, the pulse repetition rate of thepulse generator 10 may be 6.25 kHz, which provides an interval of 160 μsbetween successive pulses. Thus, pulses 110 arrive at the rate of 6.25kHz and so do the output pulses 121 from MOPA 100 which therefore alsohave an interval of 160 μs between successive pulses. For this example,the delay intervals T2, T3, and T4 may be 40, 80, and 120 μs,respectively. Therefore, the trigger pulses 210, 310, and 410, whilehaving the same repetition rate of 6.25 kHz as trigger 110, arrive withdelays of 40, 80, and 120 μs, respectively, after triggers 110. As aresult, the laser pulses 221, 321, and 421 from MOPAs 200, 300, and 400also arrive with delays of 40, 80, and 120 μs, respectively, after thelaser pulses 121 from MOPA 100. Therefore, when the output laser pulsesfrom all four MOPAs are considered in combination, they effectivelyprovide a train of laser pulses that arrive every 40 μs. The combinedsystem is therefore equivalent to a pulsed laser source that has arepetition rate of 1/40 μs=25 kHz.

The embodiment illustrated in FIG. 6 is a specific example in which thepulse repetition rate of the composite laser system (25 kHz) is amultiple (four in this case) of the pulse repetition rate of each of theindividual laser units (6.25 kHz). Thus, the delays produced byelectronic delay units 260, 360, and 460 (40, 80, and 120 μs,respectively) cause the pulses of the combined MOPA units to be equallyspaced with a time interval of 40 μs between successive pulses. Notethat in the illustration of FIG. 5, the delays produced by electronicdelay units 260, 360, and 460 have been denoted as generalizedquantities T2, T3, and T4, respectively. In a different embodiment, thedelays T2, T3, and T4 can be made variable, which will enable MOPA units200, 300, and 400 to emit their laser pulses at any desired timeinstants. Such an embodiment is illustrated in FIG. 7, in which thevariable delays for the arrival times of the pulses from MOPA units 200,300, and 400 with respect to the arrival times of the pulses from MOPAunit 100 are denoted as T_(b), T_(c), and T_(d), respectively. Note thatT_(b)<T_(e)<T_(d)<T.

In another embodiment, the timings of all the laser pulses arecontrolled dynamically by signals that are related to the patternlocations on the semiconductor wafer that are intended to be exposed bythe laser pulses. More specifically, as illustrated in FIG. 8, thepattern information 550, which is in the form of a bit map stored in adata file 500, is converted by a control computer 600 into timingsignals 610, 620, 630, and 640 using a suitable algorithm. These timingsignals are sent to the pulse generator 10 and the variable delay units260, 360, and 460. The result is that the laser pulses 121, 221, 321,and 421 from MOPA units 100, 200, 300, and 400, respectively, arrive atthe spatial light modulator array which directs them to thesemiconductor wafer at timing instants that are determined for optimizedexposure of the desired pattern.

In another embodiment, shown in FIG. 9, the control computer 600 mayprovide the desired trigger signals directly (i.e., without the need forseparate delay units 260, 360, and 460) with the proper time delaysbetween them. These trigger signals 615, 625, 635, and 645, may be sentto the MOPA units 100, 200, 300, and 400, respectively, to trigger theirfiring which results in output pulses 121, 221, 321, and 421.

An embodiment illustrating the implementation of thehigh-repetition-rate laser source in a maskless lithography system isshown in FIG. 10. The high-repetition-rate laser source, as discussedwith reference to FIGS. 4-9, is denotes as 700 in FIG. 10. As describedin the preceding paragraph, the pattern information 550 is sent fromdata file 500 to control computer 600, which converts the bit-map-formatpattern information into timing signals 610, 620, 630, and 640, whichare sent to the provides high-repetition-rate laser source 700, whichprovides laser pulses 121, 221, 321, and 421, which are sent to thespatial light modulator array 800 through illumination optics 750. TheSLM, receiving signals 650 from control computer 600, directs light rays810 into projection lens 850, which directs them onto semiconductorwafer 900 mounted on scanning stage 950. The control computer 600, inaddition to processing the pattern information for providing signals tolaser source 700 and SLM array 800, also optimally controls projectionlens 850 and scanning stage 950 for their desired operation.

The above embodiments are just a few examples to illustrate thedisclosed invention. Numerous other variations that fall within thescope of the invention are possible and will be evident to those skilledin arts of semiconductor manufacturing, lithography, signal processing,and related fields.

1. The method of generating pulses of laser radiation at high repetitionrates, comprising the steps of: (a) providing two or more laser units,each capable of producing pulses of laser radiation at a repetition rateof F (pulses per second), and capable of being triggered by externallow-voltage pulse triggers; (b) providing an electronic pulse generatorunit, capable of producing low-voltage trigger pulses that have arepetition rate of 2×F or greater, and that are capable of triggeringsaid laser units of step (a); (c) triggering one of said laser units ofstep (a) by one sub-set of trigger pulses produced by said pulsegenerator of step (b); triggering another of said laser units of step(a) by another sub-set of trigger pulses produced by said pulsegenerator of step (b); and triggering other laser units of step (a) byother sub-sets of trigger pulses produced by said pulse generator ofstep (b); (d) providing a timing data file having the timing instants atwhich all the laser pulses from all the said laser units of step (a) areto be emitted; and (e) using the timing data file of step (d) to controlthe timings of the generation of the different sub-sets of triggerpulses produced by said pulse generator of step (b), such that saiddifferent laser units of step (a) are triggered by said sub-sets oftrigger pulses produced by said pulse generator of step (b) at specifiedtime instants, such that the repetition rate of the aggregate train oflaser pulses from all said laser units of step (a) is greater than saidrepetition rate of F of a single laser unit, and the laser pulses insaid aggregate train of laser pulses are emitted at specified timeinstants.
 2. The method of generating pulses of laser radiation at highrepetition rates according to claim 1, further comprising the steps of:(d.1) providing control means; and (e) using the timing data file ofstep (d) and said control means of step (d.1) to control the timings ofthe generation of the trigger pulses of the different sub-sets oftrigger pulses produced by said pulse generator of step (b), such thatsaid different laser units of step (a) are triggered by said sub-sets oftrigger pulses produced by said pulse generator of step (b) at specifiedtime instants, such that the repetition rate of the aggregate train oflaser pulses from all said laser units of step (a) is greater than saidrepetition rate of F of a single laser unit, and the laser pulses insaid aggregate train of laser pulses are emitted at specified timeinstants.
 3. The method of generating pulses of laser radiation at highrepetition rates according to claim 1, wherein: the trigger pulses ofeach of said sub-sets of trigger pulses are produced at a regularinterval of T (seconds) with a pulse repetition rate of F (pulses persecond) providing T=1/F.
 4. The method of generating pulses of laserradiation at high repetition rates according to claim 3, wherein: thetrigger pulses of the second sub-set of trigger pulses are producedafter a delay of T2 (seconds) after the trigger pulses of the firstsub-set of trigger pulses wherein T2 is less than T and the ratio T/T2is an integer equal to two or greater; the trigger pulses of the thirdsub-set of trigger pulses are produced after a delay of T3 (seconds)after the trigger pulses of the first sub-set of trigger pulses whereinT3 is less than T and T3=2×T2; and so on for all other sub-sets oftrigger pulses such that: the trigger pulses of the n^(th) sub-set oftrigger pulses are produced after a delay of Tn (seconds) after thetrigger pulses of the first sub-set of trigger pulses wherein Tn is lessthan T and Tn=(n−1)×T2 is an integer equal to two or greater.
 5. Themethod of generating pulses of laser radiation at high repetition ratesaccording to claim 1, wherein each of said laser units is an excimerlaser.
 6. The method of generating pulses of laser radiation at highrepetition rates according to claim 1, wherein each of said laser unitsis an Argon Fluoride excimer laser operating at a wavelength of 193nanometers.
 7. The method of generating pulses of laser radiation athigh repetition rates according to claim 1, wherein each of said laserunits is a Krypton Fluoride excimer laser operating at a wavelength of248 nanometers.
 8. The method of generating pulses of laser radiation athigh repetition rates according to claim 1, wherein each of said laserunits is an excimer laser system comprising a master-oscillator sub-unitand a power-amplifier sub-unit.
 9. The method of generating pulses oflaser radiation at high repetition rates, comprising the steps of: (a)providing at least two laser units, each capable of producing pulses oflaser radiation at a repetition rate of F (pulses per second), andcapable of being triggered by external low-voltage pulse triggers; (b)providing an electronic pulse generator unit, capable of producinglow-voltage trigger pulses that have a repetition rate of 2×F orgreater, and capable of triggering said laser units of step (a); (b.1)providing at least one electronic delay unit, capable of accepting thepulses from said pulse generator of step (b) as inputs and providingoutput pulses that are delayed from said inputs by specified timeinstants; (c) triggering one of said laser units of step (a) by onesub-set of trigger pulses produced by said pulse generator of step (b);delaying another sub-set of trigger pulses from said pulse generatorusing one of said electronic delay units of step (b.1); triggeringanother of said laser units of step (a) by said delayed pulses producedby said electronic delay unit; and triggering other laser units of step(a) by other delayed trigger pulses produced by other electronic delayunits to which inputs are other sub-sets of trigger pulses from saidpulse generator of step (b); (d) providing a timing data file having thetiming instants at which all the laser pulses from all the said laserunits of step (a) are to be emitted; and (e) using the timing data fileof step (d) to control the delays of said electronic delay units of step(b.1) and the timings of the generation of the different sub-sets oftrigger pulses produced by said pulse generator of step (b), such thatsaid different laser units of step (a) are triggered by said sub-sets oftrigger pulses produced by said pulse generator of step (b) and saidelectronic delay units of step (b.1) at specified time instants, suchthat the repetition rate of the aggregate train of laser pulses from allsaid laser units of step (a) is greater than said repetition rate of Fof a single laser unit, and the laser pulses in said aggregate train oflaser pulses are emitted at specified time instants.
 10. The method ofgenerating pulses of laser radiation at high repetition rates accordingto claim 5, wherein the pulse repetition frequency F of each of saidexcimer laser units is in the range of 1,000-10,000 pulses per second.11. The method of generating pulses of laser radiation at highrepetition rates according to claim 8, wherein said master-oscillatorsub-units and power-amplifier sub-units are paired and are in the rangeof 2-8 pairs.
 12. The method of generating pulses of laser radiation athigh repetition rates according to claim 4, wherein each of said delaysT2, T3, . . . , Tn of said second, third, . . . , n^(th) sub-sets oftrigger pulses is in the range of 10-100 microseconds.
 13. The method ofgenerating pulses of laser radiation at high repetition rates accordingto claim 1, wherein the repetition rate of the aggregate train of laserpulses from all said laser units is in the range of 5,000-100,000 pulsesper second.
 14. The method of exposing a pattern on a photosensitivesubstrate, comprising the steps of: (a) providing at least two laserunits, each capable of producing pulses of laser radiation at arepetition rate of F (pulses per second), and capable of being triggeredby external low-voltage pulse triggers; (a.1) providing multiple lightmodulators and at least one optical system capable of directing saidpulses of laser radiation produced by said laser units of step (a) ontosaid photosensitive substrate; (b) providing an electronic pulsegenerator unit, capable of producing low-voltage trigger pulses thathave a repetition rate of 2×F or greater, and that are capable oftriggering said laser units of step (a); (c) triggering one of saidlaser units of step (a) by one sub-set of trigger pulses produced bysaid pulse generator of step (b); triggering another of said laser unitsof step (a) by another sub-set of trigger pulses produced by said pulsegenerator of step (b); and triggering other laser units of step (a) byother sub-sets of trigger pulses produced by said pulse generator ofstep (b); (d) providing a pattern data file having the information forexposing the specified pattern on the substrate; (e) providing controlmeans; (f) providing an algorithm to convert said pattern information ofstep (d), using said control means of step (e), into a timing data filehaving the timing instants at which all the laser pulses from all thesaid laser units of step (a) are to be emitted; (g) using the timingdata file of step (f) to control the timings of the generation of thedifferent sub-sets of trigger pulses produced by said pulse generator ofstep (b), such that said different laser units of step (a) are triggeredby said sub-sets of trigger pulses produced by said pulse generator ofstep (b) at specified time instants, such that the repetition rate ofthe aggregate train of laser pulses from all said laser units of step(a) is greater than said repetition rate of F of a single laser unit;and the laser pulses in said aggregate train of laser pulses are emittedat specified time instants and are directed by said light modulators tosaid substrate at specified time instants.
 15. The method of exposing apattern on a photosensitive substrate according to claim 14, whereineach of the light modulators is a digital micromirror device of a sizein the range of 2-50 micrometers.
 16. The method of exposing a patternon a photosensitive substrate according to claim 14, wherein each of theoptical systems is a projection lens with an object-to-image reductionratio in the range of 50-500.
 17. The method of exposing a pattern on aphotosensitive substrate according to claim 14, wherein said algorithmto convert said pattern information of step (d) into a timing data filehaving the timing instants at which all the laser pulses from all thelaser units of step (a) are emitted is such that the exposure of theentire substrate with the entire pattern data file is carried out withcost and performance optimization of the patterning exposure process.18. A laser radiation emission system capable of generating pulses oflaser radiation at high repetition rates, comprising: (a) at least twolaser units, each capable of producing pulses of laser radiation at arepetition rate of F (pulses per second), and capable of being triggeredby external low-voltage pulse triggers; and (b) an electronic pulsegenerator unit, capable of producing low-voltage trigger pulses thathave a repetition rate of 2×F or greater, and that are capable oftriggering said laser units of (a); wherein: (c) one of said laser unitsof (a) is triggered by one sub-set of trigger pulses produced by saidpulse generator of (b); another of said laser units of (a) is triggeredby another sub-set of trigger pulses produced by said pulse generator of(b); and other laser units of (a) are triggered by other sub-sets oftrigger pulses produced by said pulse generator of (b); and furthercomprising: (d) a timing data file having the timing instants at whichall the laser pulses from all the said laser units of (a) are to beemitted; wherein: the timing data file of (d) is used to control thetimings of the generation of the different sub-sets of trigger pulsesproduced by said pulse generator of (b), such that said different laserunits of (a) are triggered by said sub-sets of trigger pulses producedby said pulse generator of step (b) at specified time instants, suchthat the repetition rate of the aggregate train of laser pulses from allsaid laser units of (a) is greater than said repetition rate of F of asingle laser unit, and the laser pulses in said aggregate train of laserpulses are emitted at specified time instants.
 19. A laser radiationemission system capable of generating pulses of laser radiation at highrepetition rates according to claim 18, further comprising: (d.1)control means; wherein: the timing data file of (d) is used by saidcontrol means of (d.1) to control the timings of the generation of thetrigger pulses of the different sub-sets of trigger pulses produced bysaid pulse generator of (b), such that said different laser units of (a)are triggered by said sub-sets of trigger pulses produced by said pulsegenerator of (b) at specified time instants, such that the repetitionrate of the aggregate train of laser pulses from all said laser units of(a) is greater than said repetition rate of F of a single laser unit,and the laser pulses in said aggregate train of laser pulses are emittedat specified time instants.
 20. A laser radiation emission systemcapable of generating pulses of laser radiation at high repetition ratesaccording to claim 18, wherein: the trigger pulses of each of saidsub-sets of trigger pulses are produced at a regular interval of T(seconds) with a pulse repetition rate of F (pulses per second)providing T=1/F.
 21. A laser radiation emission system capable ofgenerating pulses of laser radiation at high repetition rates accordingto claim 20, wherein: the trigger pulses of the second sub-set oftrigger pulses are produced after a delay of T2 (seconds) after thetrigger pulses of the first sub-set of trigger pulses wherein T2 is lessthan T and the ratio T/T2 is an integer equal to two or greater; thetrigger pulses of the third sub-set of trigger pulses are produced aftera delay of T3 (seconds) after the trigger pulses of the first sub-set oftrigger pulses wherein T3 is less than T and T3=2×T2; and so on for allother sub-sets of trigger pulses such that: the trigger pulses of then^(th) sub-set of trigger pulses are produced after a delay of Tn(seconds) after the trigger pulses of the first sub-set of triggerpulses wherein Tn is less than T and Tn=(n−1)×T2 is an integer equal totwo or greater.
 22. A laser radiation emission system capable ofgenerating pulses of laser radiation at high repetition rates accordingto claim 18, wherein each of said laser units is an excimer laser.
 23. Alaser radiation emission system capable of generating pulses of laserradiation at high repetition rates according to claim 18, wherein eachof said laser units is an excimer laser system comprising amaster-oscillator sub-unit and a power-amplifier sub-unit.
 24. A laserradiation emission system capable of generating pulses of laserradiation at high repetition rates, comprising: (a) at least two laserunits, each capable of producing pulses of laser radiation at arepetition rate of F (pulses per second), and capable of being triggeredby external low-voltage pulse triggers; (b) an electronic pulsegenerator unit, capable of producing low-voltage trigger pulses thathave a repetition rate of 2×F or greater, and capable of triggering saidlaser units of (a); and (b.1) at least one electronic delay unit,capable of accepting the pulses from said pulse generator of (b) asinputs and providing output pulses that are delayed from said inputs byspecified time intervals; wherein: (c) one of said laser units of (a) istriggered by one sub-set of trigger pulses produced by said pulsegenerator of (b); another sub-set of trigger pulses from said pulsegenerator is delayed using one of said electronic delay units of (b.1);another of said laser units of (a) is triggered by said delayed pulsesproduced by said electronic delay unit; and other laser units of (a) aretriggered by other delayed trigger pulses produced by other electronicdelay units to which inputs are other sub-sets of trigger pulses fromsaid pulse generator of (b); and further comprising: (d) a timing datafile having the timing instants at which all the laser pulses from allthe said laser units of (a) are to be emitted; wherein: the timing datafile of (d) is used to control the delays of said electronic delay unitsof (b.1) and the timings of the generation of the different sub-sets oftrigger pulses produced by said pulse generator of (b), such that saiddifferent laser units of (a) are triggered by said sub-sets of triggerpulses produced by said pulse generator of (b) and said electronic delayunits of (b.1) at specified time instants, such that the repetition rateof the aggregate train of laser pulses from all said laser units of (a)is greater than said repetition rate of F of a single laser unit, andthe laser pulses in said aggregate train of laser pulses are emitted atspecified time instants.
 25. An exposure system for exposing a patternon a photosensitive substrate, comprising: (a) at least two laser units,each capable of producing pulses of laser radiation at a repetition rateof F (pulses per second), and capable of being triggered by externallow-voltage pulse triggers; (a.1) multiple light modulators and at leastone optical system capable of directing said pulses of laser radiationproduced by said laser units of (a) onto a photosensitive substrate; and(b) an electronic pulse generator unit, capable of producing low-voltagetrigger pulses that have a repetition rate of 2×F or greater, and thatare capable of triggering said laser units of (a); wherein: (c) one ofsaid laser units of (a) is triggered by one sub-set of trigger pulsesproduced by said pulse generator of (b); another of said laser units of(a) is triggered by another sub-set of trigger pulses produced by saidpulse generator of (b); and other laser units of (a) are triggered byother sub-sets of trigger pulses produced by said pulse generator of(b); and further comprising: (d) a pattern data file having theinformation for exposing the specified pattern on the substrate; (e)control means; (f) an algorithm to convert said pattern information of(d), using said control means of (e), into a timing data file having thetiming instants at which all the laser pulses from all the said laserunits of (a) are to be emitted; wherein: the timing data file of (f) isused to control the timings of the generation of the different sub-setsof trigger pulses produced by said pulse generator of (b), such thatsaid different laser units of (a) are triggered by said sub-sets oftrigger pulses produced by said pulse generator of (b) at specified timeinstants, such that the repetition rate of the aggregate train of laserpulses from all said laser units of (a) is greater than said repetitionrate of F of a single laser unit; and the laser pulses in said aggregatetrain of laser pulses are emitted at specified time instants and aredirected by said light modulators to said substrate at specified timeinstants.
 26. An exposure system for exposing a pattern on aphotosensitive substrate according to claim 25, wherein: said algorithmof (f) to convert said pattern information of (d) into a timing datafile having the timing instants at which all the laser pulses from allthe laser units of (a) are emitted is such that the exposure of theentire substrate with the entire pattern data file is carried out withcost and performance optimization of the patterning exposure process.27. An exposure system for exposing a pattern on a photosensitivesubstrate according to claim 25, wherein: said control means of (e) andsaid algorithm of (f) are operatively interrelated to said laser unitsof (a), said light modulators and optical systems of (a.1), saidelectronic pulse generator of (b), and said pattern data file of (d),such that said control means of (e), using said pattern data file of (d)and said algorithm of (f), is capable of directing the operation of saidelectronic pulse generator of (b), said laser units of (a), and saidlight modulators and optical systems of (a.1), such that the exposure ofthe entire substrate with the entire pattern data file is carried outwith cost and performance optimization of the patterning exposureprocess.
 28. An algorithm for determining the triggering instants of thepulses of at least two laser units for exposing a pixelated pattern on aphotosensitive substrate in an exposure system having multiple lightmodulators for directing the pulses from the laser units to thesubstrate, the algorithm comprising at least two input data groups andat least one output data group, wherein: (a) the specified locations ofthe pattern pixels are an input data group to the algorithm; (b) thespecified timing instants of the pattern pixels are another input datagroup to the algorithm; and (c) the timing instants for triggering thepulses of the laser units are an output data group; and the algorithmdetermines the output data group of (c) such that the exposure of theentire pixelated pattern on the entire substrate with pulses from allthe laser units is carried out with cost and performance optimization.29. An algorithm for determining the triggering instants of the pulsesof at least two laser units for exposing a pixelated pattern on aphotosensitive substrate in an exposure system having multiple lightmodulators for directing the pulses from the laser units to thesubstrate, according to claim 28, wherein additionally: (d) theconfigurations of the light modulators are another output data group;and the algorithm additionally determines the output data group of (d)such that the exposure of the entire pixelated pattern on the entiresubstrate with all the laser units is carried out with cost andperformance optimization.